A backpack is, in its simplest form, a cloth sack carried on one's back and secured with two straps that go over the shoulders and below the armpits. Backpacks are often preferred to handbags for carrying heavy loads for long periods of time, because the shoulders are better suited for bearing heavy weights for long periods of time than the hands are. Backpacks, also referred to as packs, typically stand about 3 feet (1 m) tall; and have a content between 60 and 100 liters.
Packs essentially come in two main types: externally framed and internally framed. The external frame, the more traditional of the two, uses a rigid frame which is strapped on the back and in turn carries and supports a cloth sack and potential strapped on items. Wooden pack frames have been used for centuries around the world, and metal versions first appeared in the mid-20th century. Modern pack frames are usually made from lightweight aluminum tubes.
The external frame typically has a system of straps and pads to keep the sack and the frame from contacting the body. The open structure has the added benefit of improved ventilation and decreased sweatiness. The fabric part of the pack occupies part of the frame's length, but the frame typically protrudes above and below. These areas of the frame allow bulky items (such tents, sleeping bags, and thermal pads) to be strapped on. Thus the main compartment is smaller than that of an internal-frame pack, because bulky items (tents, sleeping bags, thermal pads) are strapped to the parts of the frame not occupied by the main compartment itself.
The internal-frame pack, on the other hand, has a large cloth part in which a small frame is integrated. This frame can consist of strips of either a metal or specially designed polymer that molds to one's back to provide a good fit. Usually a complex series of straps works with the frame to distribute the weight and hold it in place. Internal-frame packs may provide a few lash points, but as the frame is fully integrated and not available on the outside, it is difficult to lash a large, heavy item so that it stays fixed and does not bounce, so most cargo must fit inside.
Internal-frame packs originally suffered from smaller load capacity and less comfortable fit during steady walking, but newer models have improved greatly in these respects. In addition, because of their snug fit, they ride better in activities that involve upper-body movement; such as scrambling over rocky surfaces. The improved internal frame models have largely replaced external frame backpacks. In Europe hardly any external-frame models are sold anymore, but in the United States, some manufacturers continue to produce them.
Large backpacks, used to carry loads over 10 kg, usually offload the largest part (up to about 90%) of their weight onto padded hip belts; leaving the shoulder straps mainly for stabilizing the load. This improves the potential to carry heavy loads, because the hips are even stronger than the shoulders, and increases agility and balance, because the load rides nearer the person's own center of mass.
However, both athletic backpacks and packs with hip belts do not account for some movement required by certain activities. In other words, such packs can be limiting on the flexibility and movement of the pack wearer, or even in the extreme, restrictive of movement.